Process of producing vacua



July 17, 1951 ALEXANDER 2,560,913

PROCESS OF PRODUCING VACUA Filed Jan. '7, 1948 IN V EN TOR.

A TTOR NEYS.

Patented July 17, 1951 UNITED STATES PATENT ormcsf 2,560,913 PROC SS or PRODUCING VAQUA aul Alex nder. Be amst d Eng and Application January 7, 1948, Serial No. 918 n. reat Bri a n r 6, 947

3 Claims. 1

Th s inventi n relat s to an impr ved oe ss of produc n vacua. y h v util zati n of a vas vapour pump. whe by h a a m y b produ d r. maintained h an fiie enoy which remains high th o gh u the ran of wo k g pressur s hic mus b traversed e ore th desired gh va uum n e atta ne By a u m apour p mps I mean vacuum pumps n which n o rat ng iquid is vapouris d by he app ication o heat a oile and s, u s que t y c nde sed. the. pump n ac on bein due n r alia) to he vapour s am h s s t up. The atta nment of high eificiency throughout a wide range, of working pressur s is to be d stingu shed, rom the p vious prae ioe. w ich the pumps us d ha e o d enioien y onl through c rta n mit d ra g of work ng pr s ures. hat is a very lo working pr ssure wh n. one of operati g luid i us d and t a hi her work n pressure wh n another type of op rat ng f u d s u d- I a iound t at the m ro ed esults i icated may be obtained by producing a vapour having characteristics, difierent from those of the oper ing fluids pre iou ly s a usin his as the operating fluid in a vacuum vapour pump of usual type t su tab y m d fi dhi v po r mu t h e a high pr ssure at working t mp ra ture (i. e. the temperature at which it is produc d in the boi er) a d. at he s e t m a pre ure s l w a po e a atm spher c emperature. It is drawn from a body of vapour in a boiler space (the pressure within which is main.- tained within certain limits, as will be described), s ass d ro gh a J t in o he p in pace. where it is directed in the pumpin direction and is condensed on a surface, such as'the wall of the pu pi spa e. m nt ined at or a ut atm spheric temperature, at which point it will have a particularly low pressure. I have found after many experiments that water-free glycerine Vapour answers the above requirements and is well adapted for' use in my improved process.

The object or the invention is the provision of an improved process of the character referred to, having the features hereafter to be described. v

It should be understood that in typical vacuum vapour pumps the operating liquid is evaporated in a boiler and the vapour is led to an annular jet from which it issues into the pumping space. Above the jet the pumping space leads tov the mouth or inlet of the pump, which is located immediately above the body of the j t and the chamber to be evacuated is, connected to this inlet. Below the jet the pumping space is conhe ted with a backing pump. d ed o ma I t n h pump g sp e a a pr ss re h ch is of the order of one millimetre of mercury orless. h pu p n s ace is sur unded b a w t cooled wall on which the vapour condenses; and the condensed liquid flows down the wall and returns to the liquid in the boiler through a liquid a w ch s uts o f the bo r from the umpin pace e ep or e pa s ge thr u e je ne of the most important characteristics of a vapour vacuum pump is its pumping speed. This is defined as the volume of gas (generally air), measured a the press e n t e mouth o the p mp. which passes p r un o t me from the space to be e a uated. hr ug th pump-t the outlet.

I prese t p a ce the op ra g liquid fo a uum vapour pumps is either mercury or certain special oils with, very low vapour pressures. One of the e. specia oi s m st om on y us d in Ensl nd ecomp ses, a a empera re f l h y above 0 0., at whi h te p ratu e ts vapour pres u e ma w namely. about 1 mm. of mercury. Its vapour pressure at, 15 C. is below 1 mmof m cury.

The best obtainable pumping speed at mouth pressur s o the o der o 1 and 1.0- m f er ury have hitherto en bt in d y vacu m pour pumps si mercury as the operat li uid- The owes tot l pressur bt inab in. a u p when u g ercury s determ ne y the vap ur pre sure o the m rcury. which is 7X 0 mm. a 15 C. By using the special oils, total pressures of 10- mm. and below can be, attained with good pumping speeds. These oils were developed larg ly a the resu of the. the ies enun d by Giaede n re ere c to he d ii sio principle of op r i s f r e am l G e, Z, tech. Phys. l, p. 337 (1923). According to Gaed t e p ure f the v p u in the re ion where it l aves the noz le o the o l r. h s to be such that the mean free, path of the vapour molecules is of the same order of magnitude as the width of the throat (or the passage between the nozzle and the outer wall) or the pump. Obviously, a hi h um i s eed is o y p s i with a wid throat, and the o e t v pou n th iet (whe e th p ra u e is n the kng te pera ure) should e at th l w st p i pressure at wh ch. it till op rates ce ta n min mum pressure is of cour e. neces y to p vent he as or ai di si b k r u h the vapour into the space hich s being vacuat d- Thus from this theory, which has been generally accepted! t ould o w tha a g o ump fluid should hav a comparatively low vapour pressure at the working temperature, and it has been generally accepted that the best pump fluids should have very low vapour pressure throughout the working range of temperature, that is, from the working temperature down to the temperature of the cooling walls of the pump. This led eventually to the development of the Apiezon oils and other oils with similar vapour pressure values.

Pump using the special oils however, although eflicient at mouth pressures of the order of 10- and 10- have low pumping speeds at mouth pressures of the order of and are almost inoperative at mouth pressures of the order of 10- and above. It ha thus not been possible for a single pump and pumpin fluid to work over ranges of mouth pressure from the region of 10 to 10- and above, and at the same time provide pumping speeds at the lower pressures comparable with those obtainable when using pumping fluids which only work effectively at the lower part of the pressure range.

This is a serious disadvantage when it is desired to produce vacua with pressures of the order of 5x 10 and below, since the pressure has to be reduced through the region of 10- and 10- to attain the higher vacua. Similarly, when it is desired to maintain a vacuum of the order of 5 10 and below, the pressure has first to be lowered through the range of 10- and 10- to the desired pressure. It is, however, an especially serious disadvantage in most manufacturing process in which gas is generally evolved after the pressure has been reduced, this gas often causing a substantial rise in the mouth pressure until it has been pumped away. This matter will be referred to more fully hereafter, but it may be noted at this point that there have heretofore been no pumps available with a sufficiently high pumping speed at pressures of the order of 10- mm. of mercury, to maintain low pressiire in systems in which great quantities of gases or vapours are continuously liberated.

It is thus of the utmost importance in practice that a pump should have a high pumping speed over a range of mouth pressures extending well above the pressure to be attained or maintained.

I have now obtained results of the desired character by proceeding along lines quite opposed to those laid down by Gaedes theory, and indeed believe that I have demonstrated that Gaedes relation between the pressure of the vapour and the width of the throat is not valid. I have found, by experiment, that for a given pump the essential requirements for eflicient working are as follows: for working in the 5X1O- to 10- range of mouth pressures a low vapour pressure at room temperature is required, and this efficiency is not impaired by the fact that the liquid has a high vapour pressure at working temperature, in spite of the fact that the vapour emerges into the pumping space substantially at working temperature. On the other hand, the relevant characteristic of an operating liquid for efiicient working in the 10- to 10- range of mouth pressures appears to be primarily a high vapour pressure at working temperatures. From the above it would seem that the ideal liquid for emcient operation throughout the working range, 10- to 10- should have a relatively high vapour pressure at working temperatures and a relatively low Vapour pressure at room temperature.

Mercury has a vapour pressure of about 17 mm. at 200 C., and a Vapour pressure at room temperature (say, C.) of 7 10 while the corresponding pressures for the special oils are of the order of 1 mm. and below 10* respectively. It would therefore appear that the vapour pressure at the jet of what I consider the ideal fluid, should be at least as high as is practicable when using mercury, and preferably higher, while at room temperature the vapour pressure of the desirable fluid should be substantially lower-than that of mercury.

I have confirmed the above suggested requirements for efliciency, and have discovered that glycerine (substantially water-free) answers particularly well to these conditions and is eminently suitable for use as the operating liquid. The vapour pressure of glycerine at atmospheric temperature is not, so far as I am aware, given in any of the published collections of data on vapour pressures. It is however, known that it has a vapour pressure at 200 C. of 46 mm. (compared lit to the corresponding vapour pressure of mercury of 17 mm.), while I have found that its vapour pressure at 15 C. is below 10 and probably about 5x 10- Its vapour pressure at room temperature is therefore substantially lower than that of mercury which at 15 C. is '7 10 I have found that, operating as will be described, with glycerine as the operating liquid, I can obtain the good pumping speed and high efilciency obtainable with the special oils at the low pressures of 10 to 10- mm. and, at the same time, the good pumping speed and high efiiciency obtainable with mercury at the higher pressures.

The use of glycerine as the operating liquid requires, for best operation, that the liquid seal, through which the condensed liquid returns from the pumping space to the boiler, should be high enough to withstand a considerable pressure diff ference, of at least 20 mm. of mercury and preferably 40 mm. of mercury, so that it should have a height of some 20 to 40 cm. of glycerine. This is of course, because of the low specific gravity of glycerine compared with mercury and its high vapour pressure at working temperatures. The height of seal used with the normal type of vacuum vapour pump now in use will be insuflicient for full efficiency in my process, particularly when operating at the higher mouth pressures such as 10 since a seal a few centimetres high is sufficient when using mercury as the operating liquid, owing to the very high specific gravity of mercury, whilst with the special oils a seal of about the same height as with mercury is suf ficient, since much smaller pressures are developed in the boiler.

It was expected that difliculties would be encountered when using glycerine because of the highly hygroscopic nature of this liquid. I have in fact found that a pump using glycerin is inoperative or highly inefficient whilst the glycerine contains moisture. I have, however, discovered that the moisture in the glycerine can be eliminated by preliminary working of the pump, the pump being in fact used to vacuum-distil glycerine. I also found that at the operating temperature and under vacuum the glycerine does not reabsorb water vapour from the gases which are being evacuated sufficiently to impair the efiiciency of the pump. If the purest glycerine available commercially is used, with as little exposure to the air as possible, preliminary pumping of a period of about ten minutes with small pumps, and about one half hour, with larger pumps, only, is required.

It should also be noted thatthe pressure'of the vapour in the boiler may be varied between lower and upper limits with certain effects upon the speed and effective range of the pump, and my preferred process includes suitable adjustment of the boiler temperature and pressures, as will be described hereafter.

In order that the invention may be more clearly understood a preferred construction of pump by which the invention may be practiced and is such as has been described in my Patent No. 2,404,021 will now be described with reference to the accompanying drawing, in which the single figure represents a vertical sectional view through such a pump. Referring to the drawing, a boiler l containing glycerine as the operative liquid 2, with an electric heater in the space 3, supports the outer tube 5, forming the body of the pump, and this supports the central vapour tube 4.

The mouth or inlet of the pump is defined as the passage immediately above the jet body I, l and is indicated at 0, and the vessel to be evacuated (not shown) is connected to this inlet.

The lower portion 1 of the jet body is fixed on a tube 6? adapted to slide over the tube 4. The vapour from the liquid 2, passing up the tube 4, enters the space in the jet body 1, 7 through holes ill in the portion 7 and issues from the annular orifice 8 into the pumping space H. The upper portion 1 is adjustable in height relatively to the lower portion l by means of the screw 13 secured to it engaging a nut l4 carried by a spider IS.

The outlet of the pump is the pipe l6 connectin the space I! with the usual backing pump, which produces a pressure in the pumping space adjacent to the outlet of the order of from 0.1 to 1.0 mm. of mercury. The tube 5 is surrounded by a water jacket i! to condense the vapour, and the condensate falls into the annular trough 18. This trough forms the upper end of a seal which shuts off the space above the boiler from the space H. The trough I8 is connected with the liquid 2 in the boiler by a pipe 19 which descends below the level of the liquid. The height of the seal is the diirerence of level between the liquid 2 in the boiler and the liquid in the trough, and the head due to this height is the difference of pressures inside and outside the tube 4. The maximum height of the seal should give a head exceeding the (inference of pressures in side and outside the tube 4 at the highest boiler pressure, 1. e., pressure inside tube 4, which it is intended to employ; this seal should be able to withstand a pressure difference of at least 20 mm. of mercury, and preferably at least 40 mm. of mercury.

The outer tube 5 has parallel walls at its upper part surrounding the jet 1, and the space between the tube 5 and jet 1, of width A as indicated by the dotted line, is the annular space which has been referred to as determining in large measure the rate of evacuation of the pump. The outer tube 5 diminishes in diameter at the portion 2!] with conical walls to the lower portion, which again has parallel Walls. The extent to which the diameter is diminished and the position of the portion of the tube with nonparallel walls is determined as follows. A line is drawn from the jet orifice 8 at an angle of 15 degrees to the pumping direction C, to meet the wall of the tube 5 at a point 2!. Calling the length of this line B, B must not exceed 2.6 A.

To secure this condition, while avoiding abrupt changes in diameter of the tube 5, the diameter must be diminishin at a point about 2|, that is to say, at a point near the level of the jet orifice 8, though the diminution may continue upwards above this point. The diminution may continue below the point 2| as shown, to give the annular space H in the lowerv portion the desired width. In the drawing, B is equal to about twice A, and this is a convenient ratio.

The action of the pump will be explained by assuming, by way of examples a series of pressures in. the pump which can obtain in practice; If the pressure at the pump inlet 5 be .002 mm., of mercury and in the outlet it, .100 mm., the pressure near the letter A will be, under the conditi'ons assumed, .001 mm., and that near the letter B .0001 mm. The pressure in the space H, a little above the outlet it will be about .120 mm.,

giving a pressure difference of .020 mm., to cause aflow of air from the space H into the outlet 18. There is, therefore, a rapid rise of pressure from a point near B to a point above the outlet l6, and consequently the air just above the outlet I6 tends to flow upwards to the point near B. Upward flow can be prevented only by impact of the molecules of the mercury vapour on the air molecules, to drive them downwards against the pressure difference.

Now the density of the mercury vapour diminishes with increasing angle from the general pumping direction, indicated by the line C and it diminishes also with increasing distance from the jet orifice. The region of least density is, therefore, along the surface of tube 5, and it is along this surface that there is the greatest danger that a backward flow of air through the vapour jet may occur. Therefore it is essential for emcient pumping that the density of the vapour along a portion of the surface of the tube 5 be sufiicient to ensure the air molecules being driven downwards against the pressure difference by impact on them of the mercury molecules.

By restricting the length of the line B with reference to the width A, the density of the vapour at the surface of the tube 5 for a given air passage A is increased, and the vapour jet can act as a screen preventing any upward passage of air molecules.

The higher the pressure at the inlet 6, the higher is the pressure in the space i l, and therefore the greater the tendency of the air to flow back past the vapour jet. By means of the adjustment provided for the height or the jet orifice, the length of the line B may be decreased and the density close to the tube 5 increased, and thereby the pump may be adjusted to pump efilciently at high pressures, such as pressures above .01 mm.

An auxiliary water jacket 25 may be arranged surrounding but not touching portions of the tubes 4 and 9, located so as to prevent the vapour in the pumping space H from becoming heated by contact with the hot wall of the tube 4. The cooling fluid is circulated through the jacket by pipes 26.

In operation, the vapour of the glycerine formed in the boiler enters the jet body I, l at the high vapour pressure corresponding to the working temperature (which as stated is 46 mm. of mercury at 200 0.), and issues from the orifice 8 and by molecular collision drives air in the space ll towards the outlet 16, thus sucking air through 'the inlet 6- and evacuating a vessel connected at 6. The jet is of course, situated intermediate the inlet and outlet of the pump, and

the vapour stream, directed by the divergent nozzle in the pumping direction, impinges continuously on the water-cooled wall 5, which is maintained at about 15 C. (room temperature), the vapour being there condensed. The vapour pressure of the condensed glycerine will be, as above stated, something below 10- mm. and probably about 5 10- mm. of mercury. The backing pump, which may, for example, be a mechanical vacuum pump, maintains the pumping space adjacent to the outlet at a pressure of the order of between 0.1 and 1.0 mm. of mercury.

Note should be taken of the influence of the boiler pressure on the pumping speed of vapour pumps, glycerine having an advantage over mercury and the special oils in that the boiler pressure can, in pumps of similar design, be raised advantageously to a higher point, with glycerine than with the other liquids. Generally speaking, with any of the operating liquids, if the pressure of the vapour in the boiler iskept at the lowest level at which the pum can operate, the pump works with the greatest efficiency in the lowest range of the working pressure, that is, the pressure of air in the mouth of the pump. Therefore the maximum pumping speed is obtained at a low working pressure, the pumping speed decreasing rapidly as the working pressure increases. The pump, at this low boiler pressure, becomes inoperative at a comparatively low working pressure.

By increasing the boiler pressure the efficient working range of the pump is extended to higher working pressures. The maximum of the pumping speed is also shifted to higher Working pressures, and usually the pumping speed at the lowest working pressures decreases. Above a certain limit however, a further increase of the boiler pressure does not result in an increase of the pumping speed in any pressure range; on the contrary, a general decrease of the pumping speed takes place.

This limiting boiler pressure depends on the nature of the liquid used and on the design of the pump. As to the latter, generally speaking a multi-stage pump needs a higher boiler pressure than a single stage one, and a pump with large dimensions needs a higher boiler pressure, for the same range of working pressures, than a pump on a smaller scale.

As to the nature of the liquid used, in the case of the special oils the limit referred to is comparatively low, because the oils decompose at a temperature which is needed to maintain in the boiler a pressure of a few millimetres of mercury. Mercury, however, does not decompose, and the boiler pressure could be increased without limit, when it is the operating liquid, but for the fact that above a certain limit the heat input must be so great that the efficient cooling of the pump wall becomes difficult, and the pumping speed decreases because of incomplete condensation of the mercury vapour on the walls. Glycerine can be heated without decomposition and without condensation troubles to a temperature at which the boiler pressure is higher. Generally, with mercury, the highest convenient boiler pressure is about 20 mm., and with glycerine about 40 mm. of mercury.

It follows from the above that a method of producing high vacua efficiently with glycerine as the operating liquid consists in maintaining a high boiler pressure of from to 25 mm. of mercury, while the workin or mouth pressure is decreasing through the 10- to 10- range, and

thereafter reducing the boiler pressure, as experience indicates, to maintain the pumping speed while the 10- and lower range is being traversed. Actual optimum values of the boiler pressure best suited to different mouth pressures depend on the particular construction of the pump and must be found by trial.

As has been indicated above, it has heretofore been an especially serious disadvantage in two of the principal manufacturing processes in which vacuum vapour pumps are used, that it has not been possible for a single pump and pumping fluid to work over ranges of mouth pressure from the region of l0 to 10- and below, with good pumping speeds throughout the range. In processes of manufacturing, reconditioning and. preserving articles and substances carried out in a vacuum chamber, e. g. cathode sputtering or thermal evaporation, and in processes for the production of sealed vessels, such as electric lamps and radio valves, gas is generally evolved after the pressure has been reduced. In the first case gas is evolved as a by-product of the process being carried out under the vacuum, and in the second case gas is evolved owing to the various parts contained in the evacuated vessel being heated to drive off gas before the vessel is sealed. This gas often causes a substantial rise in the mouth pressure until it has been pumped away. i

If, for example, a chamber is being evacuated at a pressure in the pump mouth of 10- using one of the special oils, and at one moment gas is evolved more quickly than the pump can remove it, the pressure rises. If it rises to 10 the pumping speed becomes very low and if it rises to 10- the pump is almost inoperative. By raising the boiler temperature the maximum pumping speed can be obtained at a higher mouth pressure; but the special oils decompose at temperatures not much above those commonly used.

Such difficulties are avoided when glycerine is used as the operating fluid, in the manner described herein.

Inthe manufacturing of sealed hollow evacuated vessels, the latter will be associated with the pump in exactly the same well-known way as the vessel would have heretofore been associated with the mercury pump. The contained parts in a hollow vessel, e. g. the filament and support of an electric lamp bulb, are heated to drive off absorbed gas, whilst the vacuum is being maintained, so that the mouth pressure of the pump may rise temporarily, because of this gas, from below 10' mm. to something above it. To obtain the maximum pumping speed at all pressures, the temperature of the boiler is varied to maintain the maximum pumping speed as much as possible, as explained above, the boiler temperature being decreased with decreasing mouth pressure, until the gas begins to evolve (at a mouth pressure usually below 10- mm.), when the boiler temperature should be increased, temporarily, while the gas is being driven off sufficiently to maintain the pumping speed. When the process is used for the reconditioning of the article, or in preserving the vacuum of it, the same principle should preferably be employed, of adjusting the boiler temperature as required to maintain the speed. It is, however, possible to maintain the temperature of the boiler constant, once the mouth ressure has decreased below, say, 10 mm., with good effect, since I have found that, when using glycerine, the variation of pumping speed during surges of mouth pressure is not very great, even when the temperature of the boiler is not varied. Indeed, it is possible, when using glycerine, to maintain a constant boiler temperature and pressure throughout the entire evacuation of a vessel to below 10" mm. with quite fair efficiency, the boiler pressure being a compromise between that which is most effective for the upper and lower ranges of mouth pressures respectively as, for example, a boiler pressure of about 20 mm. with a single stage pump.

It may be remarked that glycerine has the further advantages of being much cheaper to use than either mercury or the special oils, and it has additional advantages over mercury, in that any residual vapour which leaks back into the vacuum chamber is less troublesome for most purposes than mercury vapour. It is also easier. to remove if necessary.

I claim:

1. A process of producing or maintaining vacua within a range of pressure of the order of 0.1 mm. to 10* or less in an enclosed space, which comprises boiling glycerine, continuously introducing the vapour thereof at a pressure of not less than 10 mm. of mercury through a jet into an enclosed pumping space which is connected through a mouth to the space to be evacuated at a point beyond the jet in one direction and which has an outlet opening at a point beyond the jet in another direction, maintaining a pressure in said pumping space adjacent to said outlet of the order of between 0.1 and 1.0 mm. of mercury by maintaining suction outwardly through said outlet opening, directing the vapour from said jet in a continuous stream in the general direction of said outlet against a surface, maintaining said surface at about room temperature to condense the vapour which strikes the same, continuously causing the removal of the condensed glycerine and its return to the body of glycerine which is being boiled, continuing the operation until water absorbed by the glycerine has been eliminated and thereafter continuing until the total pressure in the mouth is less than 5 X mm. of mercury.

2. A process of producing vacua within a range of pressure of the order of 0.1 mm. to 10 or less in an enclosed space, which comprises continuously boiling glycerine, continuously introducing the Vapour thereof through a jet into an enclosed pumping space, the temperature at which said glycerine is boiled being such as to give, at first, a vapour pressure behind the jet of the order of at least mm. of mercury, said pumping space being connected through a mouth to the space to be evacuated at a point beyond the jet in one direction and having an outlet opening at a point beyond the jet in another direction, maintaining a pressure in said pumping space adjacent to said outlet of the order of between 0.1 mm. and

1.0 mm. of mercury by maintaining suction outwardly through said outlet opening, directing the vapour from said jet in a continuous stream in the general direction of said outlet against a surface, maintaining said surface at about room temperature to condense the vapour which strikes the same, continuously causing the removal of the condensed glycerine, and continuing the operation until the total pressure in the mouth is less than 5 10* mm. of mercury while reducing the temperature at which said glycerine is being boiled, from time to time, sufiiciently to maintain the speed at which said enclosed space is being evacuated substantially constant.

3. A process of producing or maintaining vacua within a range of pressure of the order of 0.1 mm. to 10- or less in an enclosed space, which comprises boiling glycerine continuously in a boiler space, maintaining a body of vapour thereof in said boiler at a pressure of not less than 10 mm. of mercury, drawing vapour therefrom and continuously introducing the same through a jet into an adjacent enclosed pumping space which is connected to the space to be evacuated at a point beyond the jet in one direction and which has an outlet opening at a point beyond the jet in another direction, maintaining a glycerine liquid seal between said boiler space and a point in said pumping space lower than said outlet opening,

said seal having a height suflicient to withstand a pressure difierence of at least 20 mm. of mercury between said boiler and pumping spaces, maintaining a pressure in said pumping space adjacent to said outlet of the order of between 0.1 and 1.0 mm. of mercury by maintaining suction outwardly through said outlet opening, directing the vapour from said jet in a continuous stream in the general direction of said outlet against a surface, maintaining said surface at about room temperature to condense the vapour which strikes the same, said surface being situated above said seal so that glycerine condensed thereon will flow on to said seal, and continuing the operation at least until the desired high vacuum in the space to be evacuated has been attained.

PAUL ALEXANDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,338,583 Hickman et a1. Jan. 4, 1944 2,404,021 Alexander et a1. July 16, 1946 FOREIGN PATENTS Number Country Date 552,904 Great Britain Apr. 29, 1943 

